Low doses of ethionine, when incorporated into a choline-deficient diet, induce reproducible histological alterations in the liver of rats and cause an early induction of hepatocellular carcinoma. One of the unique features of this dietary regimen is the induction, during the early stages, of a massive proliferation of "oval cells" without liver cell necrosis or inflammatory cell infiltrates. The proliferation of oval cells is accompanied by a striking increase in the level of alpha1-fetoprotein in both serum and liver. A major goal of the studies proposed in this application is the isolation of "oval cells" from the liver of rats fed a choline-deficient diet containing ethionine. In order to clarify the biological nature of "oval cells", and their possible significance in the process of liver carcinogenesis, short-term and long-term cultures of "oval cells" will be established. The nature and properties of "oval cells" will be probed by means of ultrastructural and metabolic studies and will be compared with those of isolated hepatocytes. Despite the fact that oval cell proliferation is induced by virtually all known hepatocarcinogens, the origin, nature and significance of these cells has never been studied before with biochemical approaches. A second goal of the studies proposed in this application is that of establishing optimal modalities whereby a diet deficient only in choline - rather than in several lipotropic factors -enhances the process of hepatocarcinogenesis in rats.